The aftertreatment of alloy steel grades in bottom blowing converters is carried out with oxygen as the process gas, and with nitrogen and argon as the treatment gas. Such secondary steel-refining processes are known by the abbreviations MRP (Metal Refining Process), AOD (Argon-Oxygen Decarburization), UBD (Under Bottom Blowing Decarburization) and ASM (Argon Secondary Metallurgy). They serve to refine low-alloy up to high-alloy steel grades in converter types of the same name having bottom-bath nozzles, whereby the steel grades are smelted in an arc furnace. Non-alloy types of steel are not usually produced in such converters. However, in order to achieve high quality, there are manufacturers who, despite higher costs, refine non-alloy steel types in such converters even though refining in an arc furnace would be less expensive.
In this context, it is known from West German patent no. DE-PS 2,430,975 corresponding to U.S. Pat. No. Re. 29,584 to partially replace the nitrogen and the argon by mixing them with CO.sub.2. West German patent no. DE-PS 934,772 shows a process for the production of non-alloy steel in a Bessemer-Thomas converter, which is low in toxic gases. In this process, CO.sub.2 is admitted into the bath either as a gas or by adding limestone alone or else mixed with oxygen.